Philadelphia chromosome is a hybrid chromosome resulting from a chromosomal translocation in which a small portion of the long arm of chromosome 9 is transferred to the long arm of chromosome 22. This chromosomal abnormality consistently associates with human chronic myelogenous leukemia (CML). CML is a disorder of hematopoietic cells which results in marked proliferation of granulocytic cells and often megakaryocytes.
Recent studies indicate that more than 95% of CML patients as well as 15-25% of ALL (Acute Lymphocytic Leukemia) patients harbor Philadelphia Chromosome which is designated as Ph+. Molecular studies by several groups demonstrate that during the formation of Philadelphia Chromosome, a portion of the c-abl gene is translocated from chromosome 9q34 to chromosome 22q11. Significantly, this translocation disrupts two genes, c-abl of chromosome 9 and the bcr gene of chromosome 22, resulting in the generation of a new, fused gene comprising portions of bcr and c-abl.
This chimeric gene, termed bcr-abl, produces a new protein, BCR-ABL which has several unique properties and appears to be the causative agent of the cancers with which it is associated (See FIGS. 1A through 1E). BCR-ABL protein is present only in tumor cells and its synthesis in these tumor cells is believed to be related to tumorigenicity.
Presently CML and ALL patients are treated chemotherapeutically with conventional therapeutics and radiation. Such treatment is plagued by well-known side-effects and is often of limited effect. No effective treatment for these leukemias is known. Thus, other compositions and methods for treating such cancers are being sought.
Certain naturally occurring RNA molecules, called ribozymes, possess the property of self-catalyzed cleavage. This reaction is shared by a number of small circular molecules which replicate in plants, either viroid RNAs, such as the avocado sunblotch viroid (ASBV) or satellite RNAs which are dependent on helper viruses, such as the satellite RNAs of tobacco ringpost virus and lucerne transient streak virus [Haseloff et al, Nature, 334:585-591 (1988)].
Comparison of several self-cleaving RNA sequences has led to the identification of a consensus secondary structure, termed "hammerhead", containing 11-13 conserved nucleotides at the junction of three helices that are precisely positioned with respect to the cleavage site. A hammerhead of less than 60 contiguous nucleotides was found to be sufficient for rapid cleavage in the absence of any protein [D. E. Ruffner et al, Gene, 82:31-41 (1989)]. Natural catalytic centers may be formed by contiguous regions in the RNA [P. Keese et al, in Viroids and Viroid-Like Pathogens, J. S. Semancik, ed. (CRC Press, Boca Raton, Fla., 1987), pp. 1-47; A. C. Forster et al, Cell. 49:211 (1987)] or by regions separated by a large number of nucleotides [C. J. Hutchines et al, Nucleic Acids Res., 14: 3627 (1986); L. Epstein et al, Cell, 48: 535 (1987)]. Cleavage occurs 3' to the GUX triplet where X can be C, U, or A [O. C. Uhlenbeck, Nature, 328: 596 (1987); C. C. Sheldon et al, Nucleic Acids Res., 17:5679 (1989)]. The essential constituents for the hammerhead can be on separate molecules, with one strand serving as a catalyst and the other as a substrate. Furthermore, RNA catalytic sequences require the conserved cleavage domain (GUX) to serve as the compatible substrates [Haseloff et al, supra].
One such hammerhead ribozyme, consisting of three stems or helices and a catalytic center containing 11-13 conserved nucleotides (5'-GAAAC(N).sub.n GUN(N).sub.n CUGA(N)GA-3'), has been employed to cleave HIV I gag transcripts [N. Sarver et al, Science, 247:1222-1225 (1990)].
There remains a need in the art for effective therapeutic compositions and methods to treat leukemia or ameliorate its effect on a human patient.